1. Field of the Invention
The present invention relates to heat-resistant poly(phenylene ether) (hereinafter, referred to as PPE) resin compositions useful as insulation materials for printed wiring boards or the like, prepregs using the PPE resin compositions, and laminated sheets using the prepregs.
2. Description of the Related Art
Recently, along with the progress in technology for integration of semiconductor devices used in electronic devices, higher density electronic packages, and higher density wiring of printed wiring boards, as well as in junction and mounting technologies, there have been continuous advances made in electronic devices, and in particular some startling advances in the electronic devices that uses broadband, such as mobile communication devices.
Printed wiring boards, a constituent for such electronic devices, are heading toward more highly multilayer boards and more concise finer pitch wiring simultaneously. It is effective to reduce the dielectric constant of the materials used therein for raising signal transmission speed to the level needed for acceleration of information processing, and to use materials with a lower dielectric dissipation factor (dielectric loss) for reducing transmission loss.
Accordingly, PPE resins are suitable as a material for the printed wiring boards used in the electronic devices that utilize broadband, as PPE resins have favorable high frequency characteristics (dielectric characteristics) for example in dielectric constant, dielectric loss, and the like. However, PPE resins were not so far sufficiently high in heat resistance and dimensional stability. In addition, these PPE resins carry the disadvantage that they generally have a high melting point, and the use of such a PPE resin for production of prepregs for ordinary multilayer printed wiring boards often resulted in increase in melt viscosity of the prepreg, causing processing defects such as voids and scratches during production of the multilayer sheets and providing multilayer sheets not highly reliable in quality.
PPE resin compositions aimed at improvement in heat resistance and dimensional stability, prepregs using the PPE resin compositions, and laminated sheets using the prepregs were disclosed in Japanese Unexamined Patent Publication No. 8-231847. They are respectively the PPE resin compositions containing PPE, triallyl isocyanurate and nonreactive brominated compound; the prepregs using the PPE resin compositions; and the laminated sheets using the prepregs.
However, when the resin composition described in Japanese Unexamined Patent Publication No. 8-231847 was used, it was difficult to produce multilayer sheets, as the PPE per se has a high melting point and thus the melt viscosity of the resin composition is too high at ordinary heat-pressing temperature to interpose internal conductive patterned layers into the multilayer printed wiring board.
Alternatively, PPE resin compositions comprising a PPE having a smaller molecular weight and thus better fluidity of the melt resin during processing and an improved processability at ordinary heat-press temperature, the prepregs using the PPE resin compositions and the laminated sheet using the prepregs are proposed in Japanese Unexamined Patent Publication No. 2002-265777. The PPE resin compositions described therein was effective in improving the efficiency of producing multilayer sheets.
And U.S. Pat. No. 6,352,782 discloses PPEs, of which the terminal hydroxyl group is capped with the group represented by the following formula. Even though the molecular weight of the PPEs used are not definitely described at all in the EXAMPLEs therein, the number averaged molecular weight of the PPE is described as preferably less than 10,000 and in particular about 300 to 5,000 (still more preferably about 500 to 5,000).

As described above, it is effective to reduce the molecular weight of PPEs in order to raise the fluidity of melt PPE resins and thus the efficiency of prepreg production.
However, the reduction in molecular weight of PPEs is also accompanied with a problem of the decrease in heat resistance of the resulting laminated sheets. Although it may be possible to increase the amount of a crosslinking curing agent, triallyl isocyanurate, for prevention of the decrease in heat resistance, it also accompanies the decrease in relative content of the PPE having excellent dielectric characteristics, making it difficult to produce lower dielectric constant products. Incidentally, the laminated sheets prepared by using the compositions described in Japanese Unexamined Patent Publication No. 2002-265777 above have a dielectric constant of 3.5 to 3.7 (1 MHz) when combined with an E-glass cloth, and of 3.3 to 3.5 (1 MHz) when combined with an NE-glass cloth.
In this context, there exist a need for products having further lower dielectric constant, to cope with the recent drastic increase in the amount of information processed in broadband devices.